1. Technical Field
The present invention relates to transfer devices in image reproducing apparatus. In particular the invention relates to the configuration in clamshell-type image reproducing devices in which the transfer device, the image reproducing element on which toner images are developed, and the registration rollers which transport sheets to the image reproducing element at a predetermined timing are positioned with respect to one another.
2. Description of Related Art
In xerographic photocopying machines, for example, wherein the surface of a photosensitive drum is exposed in correspondence with an original to retain a latent image subsequently developed with toner, transfer devices transfer the toner image on to a sheet brought in contact with the drum. These transfer devices generally are mounted in a rectangular trough-like shield frame. The shield frame is oriented along the axial direction of, and open for discharge exposure toward, the photo-sensitive drum. A discharge wire is disposed lengthwise within the shield frame, and insulating blocks of a pair supporting either end of the discharge wire are located at opposite ends lengthwise of the shield frame. In transfer devices of this type, a high voltage is applied to the discharge wire, producing an image-transferring corona discharge on the reverse of the sheet. Consequently, the toner image superficially retained on the drum is transferred onto the obverse side of the sheet.
With regard to photocopiers, the so-called clamshell type is known, wherein the machine body is split into an upper section which can be opened and closed freely on a lower section. In the clamshell photocopier, the photosensitive drum is located in the upper body section, and the transfer device is located in the lower body section. The end of the transfer device upstream in the direction of sheet flow is supported so as to be freely pivotal on a portion of a frame of the machine body. Further, the underside of the shield frame is urged toward the photo-sensitive drum by means of a spring.
The construction herein is such that with the upper body section closed onto the lower body section, the relative positions of the photo-sensitive drum and the transfer device are determined by abutment between a housing of the drum and a portion of the transfer device. A predetermined gap is thus maintained between the photo-sensitive drum surface and the discharge wire of the transfer device.
Sheet registration rollers in this sort of photocopier configuration are provided upstream of the photo-sensitive drum in the direction of copy paper supply, wherein the transfer device supported to be freely pivotal is downstream of the registration rollers. The distance between the fulcrum on which transfer device pivots and the discharge wire is consequently short.
Therefore, should there be dimensional aberration in the contacting portions between the transfer device and the photo-sensitive drum, owing, for example, to warp in manufacture or transport of the machine/components, wear, component replacement, or slippage in the joint between the upper and lower body sections, the angles among the registration rollers, the photo-sensitive drum, and transfer device discharge wire when they are in installation will be greatly displaced from initial settings. In consequence, the spacing between the surface of the photo-sensitive drum and the discharge wire cannot be accurately maintained.
Moreover, for separating the transferred-image bearing copy sheet from electrostatic attraction to the photo-sensitive drum, in some instances charge-removing needles are provided in the transfer device further downstream in the sheet flow direction from the discharge wire. Herein, it is also necessary to control accurately the distance between the charge-removing needles and the surface of the photo-sensitive drum. On the other hand, there is the problem that not being able to make the distance between the above-mentioned transfer device pivoting fulcrum and the charge-removing needles very extensive, even a slight dimensional aberration greatly thwarts accuracy.
In order to resolve these kinds of problems, it would do to lengthen the distance between the pivoting fulcrum and the abutment portions of the photo-sensitive drum and the transfer device serving to determine their relative positioning. The presence of registration rollers upstream in the sheet flow direction, however, limits the extent to which the fulcrum could be shifted in the upstream sheet-flow direction. Yet if the fulcrum were to be relocated downstream, this would separate both the discharge wire and the charge-removing needles away from the photo-sensitive drum, disabling effective functioning. In particular with compact photocopiers, since the diameter of the photo-sensitive drum is small, there are limitations to the position in which the transfer device can be located, such that the foregoing problems are even greater.
Furthermore, wherein a paper dust removing element which presses against the surface of the registration rollers is provided upstream of the transfer device, the paper dust removing element must be maintained at constant and sufficient pressure against the registration rollers. In configuring the transfer device accordingly, it is necessary to control very carefully the dimensions of that portion of the transfer device which supports the paper dust removing element, as well as the dimensions of the transfer device itself. This is extremely difficult.